Field of the Disclosure
This disclosure is directed to sulfur particles embedded with conductive carbon and a cathode active material containing sulfur particles which can be suitable for producing cathodes of high sulfur areal loading. Thus the present disclosure is also directed to a cathode having a high areal sulfur loading for a metal ion battery and a metal ion battery containing the cathode.
Discussion of the Background
An ongoing objective in the commercial development of electric vehicles and portable electronics is to provide batteries with higher energy densities than currently available with state of the art lithium ion batteries. One approach in achievement of this objective is to couple a metal anode, such as lithium or magnesium, with a high capacity conversion cathode, such as sulfur or oxygen, without sacrificing cycle life and rate capability. Sulfur is highly attractive because it is economical, highly abundant and offers a charge capacity that is an order of magnitude higher than conventional insertion lithium ion cathodes. However, sulfur is electrically insulating and exhibits unacceptably high mass loss during cycling due to the formation of polysulfide reduction intermediates which are highly soluble in an electrolyte and do not return to the cathode during a recharge cycle.
Thus, although elemental sulfur has been under investigation as a cathode active material in conjunction with metal anodes for more than 50 years, in order to obtain viable commercial sulfur cathode energy storage and supply source, these two fundamental challenges must be overcome. The first challenge is to enhance the conductivity of elemental sulfur. Unlike commercial lithium ion cathodes (LiCoO2) which possess a high electronic conductivity and do not require significant addition of conductive additives, sulfur is an effective insulator which is 1 billion times less conductive than LiCoO2. Therefore, in order to prepare a viable and commercially useful battery based on elemental sulfur cathode active material conductive additives are included as a component of the active material composition.
The second challenge is to control the diffusion and subsequent loss of polysulfide intermediates formed during cycling. During discharge, sulfur reduces in a stepwise manner by forming a series of polysulfide intermediates which are ionic in nature and solvate easily in the electrolyte. This causes mass loss of active material upon cycling.
To date the technical approaches taken to address and solve these two fundamental challenges have resulted in diminished charge capacity in comparison to the theoretical value of sulfur such that the desired improvement is not obtained.
Thus, the result of adding high loadings of conductive additives to improve the overall electronic conductivity is low sulfur content in the cathode and corresponding reduction of energy capacity. A second problem is the slow rate of operation due to the low electronic conductivity of sulfur and the low ionic conductivity of the reduced product, Li2S. Third, the diffusion of ionic polysulfides limits cycle life due to anode passivation and mass loss from the cathode.
Extensive research efforts have been devoted to developing methods to enhance the conductivity of elemental sulfur and to control the diffusion of polysulfide intermediates formed during cycling. Conductive hosts infused with sulfur and polymer-coated sulfur composites have been studied since Nazar demonstrated infusion of sulfur into ordered mesoporous carbon. Various micro/nano carbon hosts including spheres, nanofibers, graphene oxide and carbon paper, have been investigated as conductive hosts to contain the sulfur active material (Nazar et al. Nature Materials, 2009, 8, 500-506). A microporous carbon interlayer with pore sizes matching the dimensions of the polysulfide ions has been described (Manthiram et al. Nature Communications, 2012, 3, 1166). Sulfur has also been infused into metal organic frameworks (MOF) in order to improve conductivity by interaction between the polysulfides and the MOF oxide surface (Tarascon et al. Journal of the American Chemical Society, 2011, 133, 16154-16160). Although these systems have shown some improvement in the conductivity of the sulfur cathode, diffusion of polysulfides out of the host pores continues to be a problem and limited cycle life results. Further, because a carbon matrix is employed to enhance conductivity of the sulfur capacity of the cathode is decreased due to dilution.
In U.S. application Ser. No. 14/489,597, filed Sep. 18, 2014, the present research group has described encapsulated sub-micron sulfur particles formed in the presence of a mixed hydrophilic/hydrophobic copolymer. The resulting encapsulated sulfur sub-micron core particle is coated with a membrane of layers of self-assembling conductive polymer layers, each successive layer having a charge opposite to the previous layer. Carbon black functionalized as described in U.S. Ser. No. 14/985,170, filed Dec. 30, 2015, may be dispersed in the sulfur core or associated with an outermost conductive polymer layer. However, the special functionalization of the carbon requires extra processing and involves the use of toxic and corrosive chemicals.
In U.S. Ser. No. 14/983,763, filed Dec. 30, 2015, the present research group described a sulfur active material of a hybrid particle having a core of a hybrid composite comprising at least two elements selected from sulfur, selenium and tellurium; and a coating of at least one self-assembling polymeric layer encapsulating the core.
Although each of the above described sulfur active materials has provided incremental improvement in capacity and cycle lifetime, significantly greater improvement is necessary in order to produce commercially viable metal-sulfur batteries.
Conventionally, the sulfur cathodes as described above operate at sulfur loadings of around 1 mg/cm2 and capacities greater than 1000 mAh/g are not obtained.
Thus, there is a need for a sulfur active material which allows for complete utilization of sulfur at high loadings per cm2 while having a balance of high capacity and good conductivity.
An object of the present disclosure is to provide an elemental sulfur composite which provides high sulfur loading and utilization as a cathode active material.
A second object of the present disclosure is to provide a cathode containing an active material which allows for high sulfur loading and utilization and is suitable for utility in a battery having high capacity and high cycle lifetime.
A third object of the disclosure is to provide a battery which has sufficient capacity and lifetime to be a viable commercial energy source for electronic devices.